1. Field of the Invention
The present invention relates to apparatus for and a method of treating engine exhaust gases to reduce pollutants contained therein. More specifically, the present invention concerns apparatus containing catalysts of two different types, one of which may be a xe2x80x9cclose-coupled catalyst xe2x80x9d which is free of an oxygen storage component.
2. Related Art
Motor vehicle exhaust treatment devices such as catalytic converters have conventionally been located in an underfloor position in the vehicles. However, by the time engine exhaust gases travel through an exhaust pipe to an underfloor position, they cool significantly relative to the temperature at or near the engine outlet, so there is a significant period of low conversion activity before the exhaust gases heat the catalyst to its light-off temperature. Accordingly, during the cold-start period of engine operation there is a significant discharge of unconverted exhaust gas. Increasingly stringent governmental emissions standards require, however, that cold-start emissions be reduced. In particular, the California Resource Board (CARB) has announced new ultra-low emission vehicle standards that will prohibit vehicle emissions above 0.04 grams of non-methane hydrocarbons per mile, 1.7 grams carbon monoxide per mile and 0.2 grams NOx per mile. For most motor vehicles, a large portion (up to 80%) of the hydrocarbon emissions occurs during the first phase of the U.S. Federal Test Procedure (xe2x80x9cFTPxe2x80x9d), which encompasses the cold-start period of engine operation, and which requires simulation of cold-start, warm-up, acceleration, cruise, deceleration and similar engine operating modes over a specified time period. A variety of technologies are under development to reduce cold-start hydrocarbon emissions, including the use of close-coupled catalysts as disclosed, e.g., in Ball, D. J., xe2x80x9cDistribution of Warm-Up and Underfloor Catalyst Volumesxe2x80x9d, SAE 922338. It has been reported that close-coupled catalysts, especially Pd-containing catalysts, are effective for reducing HC emissions during cold-start of the FTP cycle.
The principal function of close-coupled catalysts, also referred to as xe2x80x9cprecatxe2x80x9d and xe2x80x9cwarm-upxe2x80x9d catalysts, is to reduce hydrocarbon emissions during cold-start. Cold-start is the period immediately after starting the engine from ambient conditions. The length of the cold-start period depends on the ambient temperature, the type of engine, the engine control system and engine operation. Typically, the cold-start period is within the first two minutes after the start of an engine at ambient temperature. FTP Test 1975 characterizes cold-start as the first bag (i.e., exhaust gas sample) of the FTP driving cycle which lasts for the first 505 seconds after starting an engine at ambient temperature, which is generally considered to be 25xc2x0 C. In an exhaust apparatus comprising a close-coupled catalyst, at least part of the total exhaust system catalyst is positioned closer to the engine than a traditional xe2x80x9cunderfloor catalystxe2x80x9d. Specifically, the close-coupled catalyst is located in the engine compartment, i.e., beneath the hood and adjacent to the exhaust manifold. The close-coupled catalyst can constitute the entire catalyst mass of the exhaust treatment apparatus or it can be used in conjunction with an underfloor catalyst. The design option depends on the engine configuration, size and space available. Due to its proximity to the engine relative to the underfloor catalyst, the close-coupled catalyst receives exhaust gas at a higher temperature than the underfloor catalyst. Accordingly, the close-coupled catalyst attains its light-off temperature more quickly than an underfloor catalyst and therefore reduces emissions earlier relative to the cold-start period. On the other hand, a catalyst in a close-coupled position receives exhaust gas at operating temperatures, i.e., post-cold-start period temperatures, higher than those at which an underfloor catalyst receives the exhaust gas. As a consequence, the close-coupled catalyst must have high temperature stability, as discussed in Bhasin, M. et al, xe2x80x9cNovel Catalyst for Treating Exhaust Gases For Internal Combustion and Stationary Source Enginesxe2x80x9d, SAE 93054, 1993.
A typical underfloor motor vehicle catalyst is a three-way conversion catalyst (xe2x80x9cTWCxe2x80x9d) which catalyzes the oxidation of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides to nitrogen. TWC catalysts, which exhibit good activity and long life, typically comprise one or more platinum group metals (e.g., platinum or palladium, rhodium, ruthenium and iridium), optionally with one or more base metals, dispersed on a high-surface area, refractory oxide support, e.g., particles of high-surface area alumina, to form a catalytic material. The catalytic material is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material. High-surface area alumina support materials, also referred to as xe2x80x9cgamma-aluminaxe2x80x9d (although it usually contains other phases of alumina in addition to gamma) or xe2x80x9cactivated aluminaxe2x80x9d, typically exhibit a BET surface area in excess of 60 square meters per gram (xe2x80x9cm2/gxe2x80x9d), often up to about 200 m2/g or more. It is known to utilize refractory metal oxides other than activated alumina as a support for at least some of the catalytic components in a given catalyst. For example, bulk cerium oxide, zirconium oxide, alpha-alumina and other materials are known for such use. Many of these other materials suffer from the disadvantage of having a considerably lower BET surface area than activated alumina, but that disadvantage tends to be offset by a greater durability of the resulting catalyst.
In a moving vehicle, exhaust gas temperatures can reach 1000xc2x0 C., and such elevated temperatures cause the activated alumina (or other) support material to undergo thermal degradation caused by a phase transition with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize alumina supports against such thermal degradation by the use of materials such as zirconium oxide, titanium oxide, alkaline earth metal oxides such as barium oxide, calcium oxide or strontium oxide or rare earth metal oxides, such as cerium oxide, lanthanum oxide and mixtures of two or more rare earth metal oxides. For example, see C. D. Keith et al U.S. Pat. No. 4,171,288.
U.S. Pat. No. 4,504,598 discloses a process for producing a high temperature-resistant TWC catalyst. The process includes forming an aqueous slurry of particles of activated or gamma-alumina and impregnating the alumina with soluble salts of selected metals including cerium, zirconium, at least one of iron and nickel and at least one of platinum, palladium and rhodium and, optionally, at least one of neodymium, lanthanum, and praseodymium. The impregnated alumina is calcined at 600xc2x0 C. and then dispersed in water to prepare a slurry which is coated on a honeycomb carrier and dried to obtain a finished catalyst.
The present invention relates to an engine exhaust treatment apparatus for abating pollutants contained in the exhaust stream of the engine. The apparatus defines a flow path for the exhaust and comprises an upstream catalyst member comprising an upstream catalytic material effective for catalyzing the oxidation of hydrocarbons and comprising a platinum group metal component dispersed on a refractory metal oxide first support. The upstream catalytic material is substantially free of oxygen storage component. There is also a downstream catalyst member comprising a downstream catalytic material which is effective at least for the oxidation of hydrocarbons and which comprises one or more catalytic metal components dispersed on a refractory metal oxide support and an oxygen storage components.
According to one aspect of the invention, the upstream catalyst member may comprise a first close-coupled catalyst member and the platinum group metal component thereof may comprise a palladium component. Optionally, the upstream catalytic material may be substantially free of rhodium. However, the downstream catalytic material optionally comprises rhodium.
The downstream catalyst member may comprise an underfloor catalyst member or a second close-coupled catalyst member that may comprise a palladium component and an oxygen storage component. There may be both a second close-coupled catalyst member and an underfloor catalyst member. An underfloor catalyst member preferably comprises a three-way catalytic material.
According to one aspect of the invention, the first close-coupled catalyst member and the second close-coupled catalyst member may have different cross-sectional dimensions and may be disposed in separate canisters. Alternatively, the first close-coupled catalyst member and the second close-coupled catalyst member have like cross-sectional dimensions and may be disposed within the same canister.
According to another aspect of the invention, the first and the second catalyst members may each comprise a catalytic material comprising palladium and the underfloor catalytic material may comprise a catalytic material comprising rhodium.
Optionally, the upstream catalytic material may comprise at least one rare earth metal oxide selected from the group consisting of neodymium oxide and lanthanum oxide.
An engine exhaust treatment apparatus according to the present invention may comprise (a) a first close-coupled, catalyst member comprising a first catalytic material effective for catalyzing the oxidation of hydrocarbons and comprising a palladium catalytic component dispersed on a refractory metal oxide first support, the first catalytic material being substantially free of rhodium and oxygen storage components; (b) a second closed-coupled catalyst member comprising a second catalytic material effective at least for the oxidation of hydrocarbons; and (c) an underfloor catalyst member comprising a third catalytic material effective for three-way abatement of pollutants. At least one of the second close-coupled downstream catalyst member and the underfloor member may comprise an oxygen storage component. The first catalytic material may comprise at least one of an alkaline earth metal oxide, a rare earth metal oxide other than cerium oxide or praseodymium oxide, and zirconium oxide.
In various embodiments, the first close-coupled catalyst member may comprise from about 0.5 to about 3.5 g/in3 of activated alumina and at least about 50 g/ft3 of palladium component calculated as palladium metal and from about 0.05 to about 0.5 g/in3 of the at least one alkaline earth metal component, calculated as the oxide. Optionally, the first close-coupled catalyst member may comprise from about 0.05 g/in3 to about 0.4 g/in3 of strontium oxide; not more than about 0.5 g/in3 of zirconium oxide incorporated into the support material; and not more than about 0.5 g/in3 of at least one rare earth metal oxide selected from the group consisting of lanthanum oxide and neodymium oxide.
The first close-coupled catalyst member may comprise at least 60 g/ft3 of a platinum group metal component, e.g., from about 75 to 300 g/ft3, and the platinum group metal component may principally comprise a palladium component.
In various embodiments, the first close-coupled catalyst member may comprise from about 0.75 to about 2.0 g/in3 of activated alumina support material; and at least one component selected from the group consisting of from about 0.05 to about 0.4 g/in3 of strontium oxide; from about 0.05 to about 0.2 g/in3 of barium oxide; from about 0.025 to about 0.3 g/in3 of lanthanum oxide; from about 0.025 to about 0.3 g/in3 of neodymium oxide; and from about 0.05 to about 0.5 g/in3 of zirconium oxide.
In a particular embodiment an exhaust treatment apparatus of the present invention may comprise a first close-coupled catalyst member comprising about 300 g/ft3 palladium, 1.23 g/in3 alumina, 0.19 g/in3 lanthanum oxide, 0.1 g/in3 zirconium oxide, 0.1 g/in3 strontium oxide and 0.16 g/in3 neodymiun oxide. The apparatus may further comprise a second close-coupled catalyst member comprising about 110 g/ft3 palladium, 1.4 g/ft3, 1.4 g/in3 alumina, 0.8 g/in3 cerium oxide, 0.5 g/in3 cerium oxide-zirconium oxide composite, 0.24 g/in3 zirconium oxide, 0.2 g/in3 lanthanum oxide, 0.1 g/in3 neodymium oxide, 0.1 g/in3 strontium oxide, 0.07 g/in3 nickel oxide and 0.06 g/in3 barium oxide. There may also be an underfloor catalyst comprising about 91.9 g/ft3 palladium, 6.56 g/ft3 rhodium, 6.56 g/ft3 platinum, 1.5 g/in3 alumina, 1.7 g/in3 cerium oxide-zirconium oxide composite, 0.1 g/in3 neodymium oxide, 0.25 g/in3 strontium oxide, 0.125 g/in3 zirconium oxide, and 0.075 g/in3 lanthanum oxide.
The present invention also relates to a method for treating the exhaust gas from an engine having an exhaust gas outlet. The method comprises flowing the exhaust gas from the exhaust gas outlet of the engine through an exhaust gas treatment apparatus as defined above. The method may comprise converting at least 10% of the carbon monoxide in the exhaust gas at the at least one downstream catalyst. By way of example, the method may comprise converting at least 25%, or at least 30%, or at least 40%, of the carbon monoxide in the exhaust gas at the at least one downstream catalyst.
As used herein and in the claims, the term xe2x80x9coxygen storage componentxe2x80x9d refers to multivalent, reducible transition metal oxides of the type which is believed to be capable of taking up oxygen from an exhaust stream during relatively oxygen-rich periods and releasing oxygen into the gas stream during relatively oxygen-poor periods. Such oxygen storage components include oxides of cerium, cobalt, iron, molybdenum, nickel, praseodymium, tungsten and vanadium. As used herein and in the claims the term xe2x80x9coxygen storage componentxe2x80x9d does not pertain to any form, including oxides, of platinum group metals or to neodymium oxide or lanthanum oxide.
As used herein and in the claims, the term xe2x80x9cclose-coupledxe2x80x9d, as it pertains to a catalyst member or to a canister containing a catalyst member, refers to a position in the flow path defined by the exhaust apparatus in which, during steady state medium or high load engine operating conditions the exhaust is at a temperature of at least about 600xc2x0 C. upon initial contact with the catalyst. Typically, a close-coupled catalyst is located in the engine compartment of a motor vehicle, and is disposed in the exhaust flow path close to the exhaust outlet of the engine, for example, within about twelve inches or less from the exhaust manifold outlet along the flow path of the exhaust gas, so that the exhaust gas does not cool significantly before it comes into contact with the catalyst. Optionally, a close-coupled catalyst may be positioned at or within the exhaust manifold. Consequently, the inlet exhaust to the close-coupled catalyst is usually at a temperature of from about 600xc2x0 to 1000xc2x0 C., more usually from about 6000 to 800xc2x0 C. Optionally, a close-coupled catalyst may be positioned in the exhaust manifold itself. The term xe2x80x9cunderfloorxe2x80x9d as it pertains to a catalyst means a catalyst which is positioned in the exhaust apparatus downstream of an upstream catalyst and which, under steady state engine operating conditions, receives exhaust gas at temperatures lower than about 600xc2x0 C., usually at from about 200xc2x0 to 600xc2x0 C., more usually from about 300xc2x0 to 550xc2x0 C. Typically, an underfloor catalyst is positioned beneath the floor board of a vehicle and outside the engine compartment, although it need not necessarily be so positioned.
Reference herein and in the claims to the quantity (xe2x80x9cloadingxe2x80x9d) of particular components of a catalyst member or catalytic material are expressed as either grams per cubic foot (xe2x80x9cg/ft3xe2x80x9d) or grams per cubic inch (xe2x80x9cg/in3xe2x80x9d). These weight per unit volume units are employed to accommodate the voids provided by the gas flow passages of a xe2x80x9ccarrier memberxe2x80x9d which, as used herein and in the claims, means a body having a plurality of gas flow passages extending therethrough and on which a coating of the catalytic material is disposed. Typical carrier members are described below. The type which has a plurality of parallel gas flow passages formed therein are sometimes below referred to as xe2x80x9choneycomb-typexe2x80x9d carriers.